location /cs {
    rewrite (.*) /cs/gen;
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $remote_addr;
    proxy_pass http://cms.uno.com; # (same Nginx instance, separate server block)
}

location ~ /(?<rest_of_uri>.*) {
    return 402 "Good news, everyone, we have hit:n$host:$server_port$request_urin$http_x_testnFinal URI: $rest_of_urinuri(+args): $uri$is_args$args";
}

curl -H "X-Test: test" www.uno.com/cs

Good news, everyone, we have hit:
www.uno.com:81/cs
test
Final URI: cs
uri(+args): /cs